1. Field of the Invention
This invention relates to reducing or solving the problems presented by exposed panel fasteners and/or fastener recesses at the outer surfaces of aircraft, particularly high speed aircraft designed to have a low radar cross-section or low ability to reflect radar back to its source. Such aircraft are covered by replaceable surface panels of radar-absorbing material (RAM), which panels are bolted to underlying support members by means of fasteners, such as internally threaded Milson sleeve bolts which threadably engage threaded shanks extending outwardly from the support surface in predetermined locations corresponding to the locations of the fastener passages or bores on the surface panels. The head of each hollow, tubular Milson sleeve bolt has a recessed hexagonal opening which permits the sleeve bolts to be tightened in place by means of a hexagonal tool. The recessed hexagonal openings of the sleeve bolts represent sharp-edged surface irregularities which increase the radar cross-section of the aircraft surface.
While the Milson sleeve bolts are hollow and have a minimal exposed outer surface for the reflection of microwaves, such reflection is increased in contoured areas of the surface panels where the fasteners are recessed to expose relatively large portions of the walls of the fastener bores in the surface panels, which exposed portions also represent sharp-edged, surface irregularities which increase the radar cross-section of the aircraft.
In view of the variety of surface contours of aircraft panels, it is not possible to manufacture fasteners having exposed surface areas which will conform to the different contours of surface panels applied to different areas of the aircraft, or to different aircraft having different surface contours. Also, in view of the strength requirements and specifications of the fastening operation, and the exposure of the outer surfaces of the fasteners to high temperatures during high speed operation of the aircraft, which requires resistance to melting, oxidation and sulfidation, it is readily apparent that the problems solved by the present invention are multifaceted.
2. Discussion of the Prior Art
There are no known fasteners for solving the problems overcome by the present invention. It is known to apply surface coatings or skins over the outer surface of aircraft panels of RAM materials to minimize reflection of microwaves by the aircraft back to a receiver, and reference is made to U.S. Pat. No. 5,016,015 issued May 14, 1991.
It is also known to increase the resistance of metal aircraft panel fasteners to attract and conduct lightening by inserting non-metallic, dielectric caps or covers into the fastener bores to cover the recessed fasteners, provide a continuous aerodynamic surface and permit the surface to receive a crack-free coat of paint. The inserts may be of premolded plastic, shaped to fill the bores, or of liquid filler or sealant which flows to fill the holes. Reference is made to U.S. Pat. No. 4,630,168 issued Dec. 16, 1986.
Finally, reference is made to U.S. Pat. No. 294,524, issued Mar. 4, 1884, which has no relationship to the problems solved by the present invention but which discloses fastener assemblies having some structural features similar to the fastener assemblies of the present invention.